Specification of concepts associated with an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) (e.g., including concepts associated with Long Term Evolution (LTE) and System Architecture Evolution (SAE)) is currently ongoing within the 3rd Generation Partnership Project (3GPP). It is envisioned that E-UTRAN will initially have limited radio coverage. To provide seamless mobility, it is necessary to handover (HO) user equipment (UEs) (e.g., mobile communication devices) in the E-UTRAN to an alternative RAT (e.g., such as a Global System for Mobile communications (GSM) EDGE Radio Access Network (GERAN) or a UTRAN) with better coverage. Generally, it is also preferred for a UE, served by a second generation (2G) network (e.g., such as a GERAN) or a third generation (3G) network (e.g., such as a UTRAN), to switch to (e.g., via a handover) an E-UTRAN once the UE is within the coverage of the E-UTRAN. The E-UTRAN is preferred since higher data rates are offered by the E-UTRAN, which enables services with greater bandwidth requirements. A handover between two different RATs is referred to as an inter-RAT (IRAT) handover.
Network administrators are often responsible for multiple networks which use different RATs that cover overlapping areas at different frequencies. It is also common for at least one of the networks to cover a wider area than other networks, especially at the early stages of deployment of new networks (e.g., such as E-UTRANs).
Moving between different RATs is a demanding process for both a UE and a network. On the UE side, measuring a RAT on a different frequency requires the UE to switch modes, which prevents the UE from sending/receiving information to/from a connected RAT. In addition, when performing RAT measurements, the UE consumes more battery power than during normal operation. On the network side, scheduling needs to be adapted to the restrictions associated with the UE, which affects network efficiency. The restrictions associated with the UE and the network may reduce an end user's experience due to higher latency, a lower data rate, and/or a higher probability of dropped calls during performance of measurements.
If two or more networks have different RATs, a network administrator sets priorities between different networks in order to ensure that UEs connect to the network with the highest performance (e.g., a network with a newest and least deployed RAT). The network administrator also attempts to ensure that when a UE moves from a network area not covered by a source RAT, the UE experiences seamless mobility to a target RAT with overlapping coverage (e.g., a more established and deployed RAT).
A process for IRAT handover triggering from a first (source) RAT (e.g., a LTE network) to a second (target) RAT (e.g., a Wideband Code Division Multiple Access (WCDMA) network) may include one or more of the following steps. If a UE is connected to the first RAT and detects poor coverage of the first RAT, the UE begins IRAT handover measurements and detects the second RAT with good coverage. The UE reports the IRAT handover measurements to the first RAT network, and the first RAT network commands the UE to handover to the second RAT, after receiving an acknowledgement of an IRAT handover request. The UE connects to the second RAT network and sends a handover confirm message to the second RAT network. The second RAT network sends a handover success report to the first RAT network.
One parameter that needs to be controlled (e.g., when triggering an IRAT handover) is an absolute threshold that defines when the first RAT coverage is becoming poor and IRAT handover measurements are needed. If the setting of this threshold is not optimal, an end user's experience may be degraded. For example, if the IRAT handover measurement is triggered too late, the UE may not be able to report the IRAT handover measurements to the first RAT network or may not be able to receive the handover command due to a heavily degraded connection with the first RAT (e.g., resulting in a radio link failure). If the IRAT handover measurements are triggered too early, the UE may not find a second RAT network with good coverage or the first RAT network coverage may recover soon after the IRAT handover measurement was triggered. In both cases, the IRAT handover measurement will not be used for any purpose, a handover to a lower priority RAT may be unnecessarily triggered, or the call would be dropped (e.g., due to the more demanding task of measuring other RATs while maintaining a connection with a serving RAT) resulting in an inefficient use of the networks.
In an early stage of network deployment, coverage is provided only in hot-spots and it is important to permit UEs to move to a lower priority network with wider coverage (e.g., the IRAT handover measurements are triggered as soon as poor coverage is detected). As new networks are deployed, full coverage may be provided in most areas and it is important to ensure that IRAT handover is triggered only in edge cells (e.g., cells in the middle of a coverage area will not need to trigger IRAT handover). In some instances, cells may initially provide coverage in a hot spot, and, as the network deployment develops, the cells may end up being located far from the edges of the network. Since optimizing values of the IRAT handover parameters (e.g., per cell) is an expensive process, it is typical that default parameters are used for an entire life cycle of a cell.
In order to avoid radio link failure, network administrators often set requirements on RAT coverage too high (e.g., the threshold defining poor coverage for a RAT may be set to a too restrictive value), resulting in too early IRAT handover triggering and inefficient use of the network. Typically, it is safer to trigger the IRAT handover too early, rather than too late, because triggering the IRAT handover too late results in radio link failure.
Setting requirements on RAT coverage in such a manner may be beneficial for cells partially covering areas that need several RATs in order to provide full coverage. However, setting requirements on RAT coverage too high leads to an inefficient use of a highest priority network for centrally-located cells (e.g., where IRAT handover is unnecessary) because a UE that is handed over to a lower priority RAT will in most cases not return to the highest priority RAT until poor coverage is detected or until the UE enters an idle mode (e.g., via cell reselection).
This situation may be corrected (e.g., manually or automatically) by lowering the poor coverage threshold on cells experiencing too early IRAT handover triggering. However, detection of a too high coverage threshold is not currently possible. For example, a network may include two LTE cells (e.g., cell LTE-A and cell LTE-B). Cell LTE-A may be a border cell and may include optimal settings for IRAT handover triggering. When coverage of cell LTE-A falls below a threshold, UEs may be handed over to WCDMA network in a seamless manner. Cell LTE-B may be a center cell and may include a poor coverage threshold that is set too high. When coverage of cell LTE-B temporarily drops, UEs may be handed over to the WCDMA network unnecessarily. If the poor coverage threshold was set lower, the UEs would continue in cell LTE-B without any negative effects.
In this example, both cells (e.g., cell LTE-A and cell LTE-B) experience normal operation, no radio link failure occurs, and every IRAT handover is successfully completed. Currently, however, there is no mechanism for cell LTE-A to verify that it was a good decision to handover the UEs to the WCDMA network, and there is no mechanism for cell LTE-B to determine that the handovers of the UEs to the WCDMA network were triggered too early. As a consequence, no manual and/or automatic tuning can be performed.